For example, Patent Document 1 discloses an exhaust gas purifying apparatus for an internal combustion engine including catalyst temperature estimation means for calculating a catalyst temperature estimated value of a catalyst placed in an exhaust passage based on an operation state of the internal combustion engine. In this conventional apparatus, an estimated value correction amount is calculated from a deviation between an estimated value and measured value of an exhaust gas temperature at a catalyst outlet (hereinafter referred to as an “outlet gas temperature”), and the estimated value correction amount is used to correct the catalyst temperature estimated value.
Including the above-mentioned document, the applicant is aware of the following document as a related art of the present invention.    [Patent Document 1] Japanese Laid-open Patent Application Publication No. 2003-336538    [Patent Document 2] Japanese Laid-open Patent Application Publication No. Hei 10-196433    [Patent Document 3] Japanese Laid-open Patent Application Publication No. 2005-127285    [Patent Document 4] Japanese Laid-open Patent Application Publication No. Hei 10-159543